1. Field of the Invention
The present invention relates to a pulse arc welding method configured to ensure stable welding even if the mixing ratio of the shield gas is changed.
2. Description of the Related Art
FIG. 5 shows examples of the waveforms of welding current Iw (waveform A) and welding voltage Vw (waveform B) applied in a conventional consumable electrode pulse arc welding. As seen from the waveform A, during the peak rising period Tup (time t1-t2), transition current will flow, which rises from the base current Ib to the peak current Ip. During the same period, as seen from the waveform B, transition voltage, which rises from the base voltage Vb to the peak voltage Vp, will be applied between the welding wire and the base metal. During the peak period Tp (time t2-t3), the peak current Ip flows as shown in the waveform A, and the peak voltage Vp is applied as shown in the waveform B. During the peak falling period Tdw (time t3-t4), transition current reducing from the peak current Ip to the base current Ib will flow, and transition voltage reducing from the peak voltage Vp to the base voltage Vb will be applied. During the base period Tb (time t4-t5), base current Ib whose value is small enough not to grow droplets will flow, while base voltage Vb will be applied. The period from t1 to t5 is a pulse period Tf.
The lengths of the peak rising period Tup and the peak falling period Tdw are determined properly, depending on the material of the base metal. In the pulse MAG welding where the base metal is steel, the two periods are set to be so short that the peak current has a rectangular waveform. On the other hand, in the pulse MIG welding where the base metal is aluminum, the two periods are set to be relatively long, so that the peak current has a trapezoidal waveform. The transition current mentioned above is increased and decreased linearly. Alternatively, the transition current may be varied in a curved manner to improve the weldability, as disclosed in JP-A-2005-28383 or JP-A-2006-75890. Also, the peak current Ip may be increased stepwise, as disclosed in JP-A-2005-118872, for example. For the shield gas, use may be made of a mixture of 80% argon gas and 20% carbonic acid gas for the pulse MAG welding, or 100% argon gas for the pulse MIG welding.
In the consumable electrode arc welding, it is important to control the arc length for obtaining good welding quality. As known in the art, the average Vav of the welding voltage Vw is generally proportional to the length of an arc. Based on this fact, the control of the arc length is performed by controlling the output of the welding power source so that the welding voltage average Vav becomes equal to a predetermined voltage set value. Likewise, in the pulse arc welding, the output control of the welding power source is performed by controlling the pulse period Tf so that the welding voltage average Vav becomes equal to a voltage set value (Frequency Modulation Control). In a different way, while the pulse period Tf is kept constant, the peak period Tp is varied for performing the output control of the welding power source (Pulse Width Modulation Control).
FIG. 6 shows a “1 pulse-1 droplet” transfer range, used for setting the peak period Tp and the peak current Ip. In the figure, the horizontal axis represents the length of the peak period Tp (in milliseconds), while the vertical axis represents the peak current Ip (in amps). The shaded part shows the so-called “1 pulse-1 droplet transfer range” in which only one droplet is transferred in synchronism with the pulse period Tf. When a combination of the peak period Tf and the peak current Ip (called the “unit pulse condition”) is within the shaded range, the 1 pulse-1 droplet transfer occurs. The unit pulse condition should be within the 1 pulse-1 droplet transfer range and selected for producing good bead results (with fine appearance and no undercuts). In an instance where the peak current Ip is not constant, an integrated value of the peak current Ip over the peak period Tp is calculated, and the two parameters are adjusted so that the current integration value falls within an appropriate range corresponding to the shaded range. The 1 pulse-1 droplet transfer range will vary depending on factors such as the kind of the welding wire, the mixing ratio of the shield gas, the wire feeding speed, etc. Therefore, in accordance with the change of these factors, the unit pulse condition should be readjusted.
FIG. 7 illustrates the occurrence of an arc when the unit pulse condition is within the 1 pulse-1 droplet transfer range. Specifically, an arc 3 strikes between the base metal 2 and the tip of the welding wire 1 sent out from the welding torch 4. On the base metal 2, a molten pool 2a is formed. An arc anode point is formed above the molten part 1a at the tip of the wire. Thus, the molten part 1a is wrapped up with the arc 3. On the other hand, the arc cathode point 3b is formed on the molten pool 2a. Immediately after the ending of the passage of the peak current Ip, the transfer of a free molten droplet 1b occurs.
The setting of the unit pulse condition described above is performed on the assumption that the mixing ratio of the shield gas is a standard ratio. For example, in the pulse MAG welding with a steel base metal, the shield gas is a mixture of argon gas and carbonic acid gas. In Japan, the standard ratio in this case is 80% argon gas and 20% carbonic acid gas.
When a gas container properly adjusted to the standard ratio is used for supplying the shield gas, the welding can be performed with substantially no variation in the mixing ratio of the shield gas. However, in a large-scale factory, the argon gas and the carbonic acid gas may often be stored in separate tanks, and the two gases are mixed when needed to be supplied to a welding apparatus. In such a case, the mixing ratio of the shield gas tends to fluctuate in an early stage of the beginning of the factory operation until it becomes stable. This fluctuation can be as great as ±5 to ±10%, depending on the feeding systems of the shield gas. Further, in addition to the fluctuation in the early stage, there can be some minor fluctuation even in a stable stage, which may be no greater than about ±5%.
There is a case where the mixing ratio of the shield gas should be precisely adjusted in performing welding in light of the shape of the work or required welding quality. For such high quality welding, the standard ratio of the shield gas is set by increasing or decreasing the ratio of the argon gas. As known in the art, the arc condition is often kept stable when the ratio of the argon gas in the shield gas is increased. This is because the increase of the argon gas ratio facilitates the transfer of a molten droplet. Therefore, it is often unnecessary to reset the unit pulse condition when the ratio of argon gas is increased.
On the other hand, when the argon gas ratio decreases in the shield gas, the transfer of a droplet occurs less easily, thereby making the arc condition unstable. This phenomenon will be described below with reference to FIG. 8.
FIG. 8 illustrates the condition of the arc striking portion in an instance where the argon gas ratio of the shield gas is decreased from the standard value. Specifically, as shown in FIG. 8(A), when the argon gas ratio is reduced, the arc anode point 3a is formed at the bottom of the molten wire 1a. In this case, the neighborhood of the arc anode point 3a becomes very hot, thereby causing metal vapor 5 to gush downward from the bottom of the molten wire 1a, as shown in FIG. 8(B). As a result, the arc anode point 3a is urged upward by force 6 from the metal vapor 5, making the droplet transfer unstable. Then, with the droplet transfer prevented from occurring, the molten portion at the tip of the welding wire becomes bigger, as shown in FIG. 8(C), and a lot of sputter 7 will be produced, scattering in undesired directions.
One way to address the above-described inconvenience is to increase the peak current Ip for shifting the arc anode point 3a upward from the bottom of the molten wire 1a. However, by increasing the peak current Ip, the arc 3 flares out and the arcing strength becomes greater. As a result, more undercuts are formed, and less beads with good appearance are obtained. In addition, more sputtering will occur due to the increase of the arcing strength.